1. Field of the Invention
The present invention relates to internal release compositions as additives to synthetic resin mixtures for use in the production plastic products, such as of plastic lenses. More specifically, the present invention relates to a composition of phosphoric acid esters for the production of polyurethane or polythiocarbamate lenses for eye glasses.
2. Description of the Known Art
Lenses for eye glasses fabricated from plastic resinous substances are known in the art. Traditionally, the lenses are made from diethylene glycol bis(allyl carbonate), which is known commercially in its monomeric or polymerized form as CR-39. According to known manufacturing processes, the CR-39 monomer is mixed with free radical initiator, such as diisopropyl peroxy dicarbonate. This mixture is injected within a pair of glass mold sections that are sealed by a gasket and held together by a spring clip or other suitable fastener. The filled mold assembly is then heated in a water bath or oven to a predetermined temperature to cure the resin.
The wide acceptance of CR-39 is due to its clarity, tintability, high scratch and impact resistance, high resistance to discoloration and resistance to warping or distortion. However, CR-39 has a major drawback. The plastic lenses made from CR-39 have a refractive index (nd) of about 1.50, which is lower than the refractive index of conventional glass lenses, the latter being about 1.52. The thickness of the lenses must therefore, be larger than the thickness of conventional glass lenses.
Efforts have been made to develop synthetic materials which are capable of producing higher refractive index lenses than those currently produced by CR-39. Polythiocarbamate (hereinafter polythiourethane) resins have been known as one of the best lens resins capable of achieving high refractive index, generally about 1.6, with outstanding optical and physical lens properties. A more complete description of this type of material can be found in U.S. Pat. No. 4,689,387 and U.S. Pat. No. 4,780,522.
While casting to form a plastic lens is well known, the process for industrial production of plastic lenses, especially polyurethane, has been extremely difficult. One major drawback of molding polyurethane or polythiourethane is the tendency of these polymers to adhere to glass molds, making mold disassembly very difficult if not impossible. The adherence of the lens to the inner surface of the mold during casting was found to depend on many processing parameters or variables, such as the type of glass used to make the mold, cleanliness of the mold, age of the mold, shape and curvature of the mold, composition of the lens forming material, length and temperature of the curing cycle, and the elasticity of the gasket material used in the mold assembly. To obtain high yields and thus make industrial production possible, the adherence of the mold must be sufficient to firmly hold the plastic lens against the mold surface through the curing stage, yet sufficiently weak to allow cohesive separation following cure. Premature release before curing is completed, usually results in a defective lens surface. This phenomenon is known as the demolding defect. Conversely, too much adherence between the lens and mold after curing will result in difficult disassembly and/or mold damage. This defect is known as poor mold release.
Experience has shown that in spite of the close control of many known variables, the industrial production of polyurethane and polythiourethane plastic lenses has been very costly because of inconsistent mold release. One known method attempted to solve this problem by coating the inner surface of the mold with surface lubricants, such as siloxanes and fluorinated hydrocarbons. These are known as external mold release agents. However, the process of applying mold surface lubricants has several drawbacks including part or all of the mold release film transferring to the lens surface. This in turn may create areas of inconsistent physical properties across the lens surface, which can adversely affect the adhesion performance, and tinting of coatings applied to the lens.
Additional disadvantages of using external mold release compositions include the technical difficulties associated with this process:
1) The mold release is generally applied by dipping molds into a release solution bath. In the past, release baths could employ fluoro-chloro hydrocarbons, e.g., Freon, as the solvent. Recently, an EPA ban on such solvents dictates the use of environmentally friendly solvents, such as aqueous based systems. The aqueous systems are highly pH dependent. Changes in pH can cause agglomeration of release particles in the bath--consequently staining the mold, and/or compromising integrity of the release system. The result is at best a stained lens, at worst, a loss of molds, or both.
2) The mold release may be applied manually--by hand--similar to waxing automobile paint. Wiping the release onto the mold surface results in extra handling, and increased possibility of mold damage and/or staining.
Although integrity of the release itself is generally reliable, the process is extremely labor intensive.
Instead of coating the mold with mold release system, it has been proposed to add a releasing agent to the lens forming material and then subject the whole mixture to cast polymerization. These release systems are known as internal release agents. The known art discloses the use of silicones, fluorinated hydrocarbons, fatty acids, and ammonium salts as internal release agents in lens casting processes, see U.S. Pat. Nos.: 4,622,376 and 4,929,707 and EP 271 839. These types of internal release materials, when used in the casting process of polyurethane or polythiourethane, often fail because of poor and inconsistent releasing or due to compatibility problem, such as solubility problem or adversely affecting the physical properties of the cast lens such as optical clarity, rigidity and hardness.
U.S. Pat. No. 4,662,376 discloses that a mixture of mono and dialkyl phosphates of long straight chain alkyl groups of from 8 to 16 carbon atoms, commercially available under the trademarks Ortholeum.TM. and Zelec.TM.UN (available from DuPont, Inc., Wilmington, Del.), are suitable internal release agents for cast polymerization of acrylates and allylic carbonates in a preferred amount of 25 to about 100 ppm of the total weight of lens forming material. (U.S. Pat. No. 4,662,376 refers to the Zelec.TM.UN as having 16 to 18 carbon atoms in the alkyl group(s), however, according to the MSDS from DuPont, Zelec.TM.UN has 8 to 16 carbon atoms in the carbon chain). Casting polyurethane or polythiourethane lenses, on the other hand, requires much larger amounts of these internal release agents, about 2000 ppm, to induce consistent releasability of the polyurethane or polythiourethane lens from the mold. However, the use of amounts of 1000 ppm or more of these long chain alkyl phosphate release agents forms hazy lenses. Therefore, using these release agents alone is unsuitable for casting polyurethane and polythiourethane lens materials.
EP 271 839 describes, inter alia, the use of acidic mono or diphosphate esters including dibutyl phosphate as internal releasing agents in polyurethane compositions for lenses. U.S. Pat. No. 4,975,328 describes the use of short chain dialkyl phosphoric acid esters, such as dibutyl phosphate, as an internal release for casting polythiourethane lenses. In this patent, the use of short chain dialkyl phosphoric acid esters is described at rather high concentrations (up to 20%) and most of the examples describe the use of dibutyl phosphate at 2.5%, i.e., 25,000 parts per million. This higher concentration is due to the fact that dibutyl phosphate alone is not a very efficient releasing agent.
However, high amounts of internal release is undesirable because of the negative effect on the lens mechanical properties. Polythiourethane lenses, in general have poor high temperature performance, such as for example, lenses tending to distort during tinting. In addition, polythiourethane lenses produce strong sulfur odors during lens processing surfacing and edging!. These two inherent defects worsen and become unacceptable as the lens becomes softer due to the addition of higher amounts of internal release agent. Therefore, dibutyl phosphate alone appears to be unacceptable as an internal release agent for manufacturing polythiourethane lenses for eye glasses.
Accordingly, the need still exists for good consistent mold releasability in the industrial manufacturing of polyurethane and polythiourethane lenses. Therefore, the present inventors have conducted intensive research to develop an efficient internal release system to overcome the problems and disadvantages inherent in the prior art.